The present invention relates to the field of electrical capacitors. More specifically, it relates to the use of certain insulating fluids for the impregnation of the wound coils of self-healing capacitors having metallized plates.
By self-healing capacitors is meant in the present context capacitors which include at least one plate consisting of a very thin layer of metal of a few hundred angstroms in thickness, the main property of which is that, when the capacitor breaks down, the said plate volatilizes at the point where the said breakdown occurs. This plate may either be a thin layer of metal, such as aluminium or zinc or an alloy of these two metals, deposited directly on a polymer film, such as polypropylene or polyester, which constitutes the solid dielectric, or may be deposited on both faces of a paper, this plate then being associated with one or more polymer films which act as a dielectric. In the latter case, the doubly-metallized paper does not act as a dielectric.
The wound coils of these capacitors are advantageously impregnated with an insulating fluid. Impregnation typically takes place after the solid materials have been wound, by introducing, under reduced pressure, an insulating fluid which, at the time of impregnation, is in the form of a low-viscosity liquid. Once the impregnation has been completed, the capacitors may undergo a heat treatment. This is necessary when the insulating fluid consists of systems having 2 polycondensable and/or crosslinkable components which are to a greater or lesser extent charged with an inert liquid so as to minimize the shrinkage.
It has been observed that, when the impregnation of the metallized polymer films of the wound coils for self-healing capacitors is carried out using certain aromatic liquids, such as mixtures of mono- and dibenzyltoluene, phenylxylylethanes, the said capacitors have a very short lifetime under voltage.